[time-nuts] Phase, One edge or two? (was Digital mixing with a D Flip Flop)

WarrenS warrensjmail-one at yahoo.com
Thu Oct 23 15:24:09 EDT 2014


Lots of interesting responses,
but I did not see any posted that answered the original question:

"Is the CERN method described in the paper the best way to make a state of
the art femtosecond DDMDT?"
www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf

Restating
Assuming it is kept Digital, and not taken to the next level [analog],
by sampling more of the 10MHz waveforms than just the zero crossings.

If making a digital sampler type of tester to measure Femto second phase
differences, Is there (and should there be) a strong preference for using
one or two edges on either or both the ref and signal inputs?


Whether the second edge helps or hurts in the other cases brought up,
depends on where the signals come from and what they are being used for.

One extreme is the typical GPS timing pulse output, where the second edge
can not be used for timing.

On the other hand if a square wave signal is coming from a clean, high freq
signal that has been divide by N, then the second edge could have very
useful and needed information, such as when applied to an XOR phase
detector.

additional consideration:
Unlike a digital sampler which tend to use a single edge,
most if not all high end phase measurers I know of
averages the results at both edges of both signals.
Also anything that depends on a DMTD mixer for its operation, such as
single mixer, dual mixer, TPLL, they are all using at least both edges of 
the
signals, and depending on the degree of overdrive, they may be using a lot
more of signal that is near the zero crossing point.

ws

-----------------------------------------------------------------
>
> Phase, One edge or two? (was Digital mixing with a D Flip Flop)
> Bob Camp posted
>
> Looking at both zero crossings would give you a lot of information about
> the duty cycle of the input waveforms. If that's what you are after -
> there are easier ways to do it. If that's not what you are after, it's
> just going to mess up the readings.
>
> -------------------------------------------------------
>>
>> Poul-Henning Kamp Posted;
>> The only reason to look at both zero crossings would be to double
>> the frequency of the input signal to the loop (ie: 2Hz from a 1PPS
>> instead of a 1Hz), at the cost of adding a whole lot of noise
>> in the process.  Don't do it.
>>
>




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